DEVICE FOR WETTING AND CLEANING CYLINDERS IN AN OFFSET ROTARY PRINTING MACHINE

Abstract: Device for wetting and/or cleaning solenoids in an offset rotary printing machine having a plate cylinder, an offset cylinder and an impression cylinder. The device comprises wetting cylinders running against the plate cylinder, and a ramp with several nozzles for spraying liquid towards the wetting solenoids in atomized condition, said ramp being mounted in parallel with the rotational axes of the cylinders. A valve is provided for each nozzle having at least one separate solenoid controlled inlet for liquid and an outlet to the associated nozzle. The solenoid control inlet comprises a valve with a solenoid armature (25) which is spring biased towards the closed position of the valve, and a valve element (26, 27) displaceably mounted in the solenoid armature and spring biased towards the closed position of the valve, said valve element in this position engaging a valve seat. Abutments (30, 31) for limiting the displacement of the valve element in relation to the solenoid armature under the spring bias of the valve element are provided on the solenoid armature and the valve element, said abutments being kept separated against the spring bias of the valve element when the valve element engages the valve seat. The invention also relates to a method in operating the device wherein the solenoid current at opening of the valve after an initial increase at constant voltage over the solenoid is adjusted to a predetermined value which is maintained for a predetermined period the current then being decreased below the predetermined value to be kept at a lower value while the valve is in open position.

Description: DEVICE FOR WETTING AND CLEANING CYLINDERS IN AN OFFSET ROTARY PRINTING MACHINE The invention relates to a device for wetting and cleaning cylinders in an offset rotary printing machine of the type including a plate cylinder, an offset cylin¬ der and a impression cylinder as well as wetting cylin¬ ders running against the plate cylinder said wetting cy¬ linders being moistened from a number of nozzles in a ramp mounted in parallel to the rotational axis of the cylinders, by a moistening fluid, usually water, being sprayed from the nozzles towards one of the wetting cylinders in atomized condition. Each nozzle has an inlet for moistening fluid and an inlet for a cleaning liquid, and these inlets can be opened and closed individually by means of a solenoid valve in order to supply moistening fluid or cleaning liquid, respectively, through the nozzle. The solenoid valves are computer controlled in dependence of different operating parameters of the printing machine. The purpose of the invention is to simplify the construction of the nozzle ramp by providing a compact and economical valve arrangement wherein the valves are able to open and close rapidly without bouncing causing residual spray, and wherein the period betwen zero flow and maximum flow is as short as possible. Then, pressure drops and pressure peaks which arise at the rapid opening and closing of the valves and which cannot be neglected shall be eliminated by pressure equalization in the con- duit system to the inlets of the valves. The purpose of the invention is also to provide adjustment of the valves at high speed and high timely precision without too much energy being dissipated in the valves. In order to explain the invention in more detail reference is made to the accompanying drawings in which FIG. 1 is a side view of a valve unit according to the invention, FIG. 2 is a fragmentary vertical cross sectional view of the valve unit in FIG. 1, FIG. 3 is an enlarged cross sectional view forming part of FIG. 2, FIG. 4 is a horizontal cross sectional view of the valve unit in FIG. 1, FIG. 5 is an enlarged axial cross sectional view of a solenoid armature having a valve element of rubber or rubber-like material, FIGS. 6 to 9 are fragmentary views in reduced scale of the solenoid armature disclosing different functional positions, and FIG. 10 is a diagram illustrating different para¬ meters of the valve function. The wetting ramp can comprise a number of nozzles e.g. twelve, arranged in a row each nozzle having two valves for controlling the supply of water (moistening fluid) and cleaning liquid. to the nozzles. For each nozzle there is provided a valve housing 10, two sole¬ noids 11 being connected to said housing. The valve housing straddles two conduits 12 and 13 one for supply- ing water and the other for supplying cleaning liquid, as shown in FIG. 2, and as will more clearly seen in FIG. 3 an aperture 14 is made in conduit 12. An 0-ring 15 is provided around the aperture to seal between the valve housing and the conduit. Through the aperture the conduit is connected with a passage 16 in valve housing 10 said passage in turn communicating with a passage 17 in valve housing 10. Conduit 13 is connected in a corresponding manner. A yoke 18 and the valve housing are mounted by means of a screw connection 19 on a support rail 20. A spray nozzle 21 is mounted in the yoke, communicating with a supply passage 22 which is connected to a passage 23 in valve housing 10 at the center of this latter passage. Each end of passage 23 opens into a recess 24, and passages 17 open each into one of the two recesses. The armature 25 of each solenoid 11 has a plunger 26 displaceably guided therein, as well as a rubber element 27, FIG. 5, provided as a valve element. A compression spring 28 biases plunger 26 and armature 25, and also a compression spring 29 which is weaker than spring 28 biases the armature. By the bias of spring 28 rubber ele¬ ment 27 is kept engaged with the bottom of recess 24 and keeps the end of passage 23 closed (FIG. 6) . When the armature is attracted it moves initially against the bias of spring 29 in relation to piston 26, the rubber element 27 still being in closed position (FIG. 7) until a shoulder 30 on the armature engages a shoulder 31 on the plunger at 32. Then, the armature moves against the bias of springs 28 and 29 and lifts the plunger and the rubber element so that passage 23 will be uncovered, FIG. 8. After further movement the completely opened position according to FIG. 9 will be reached. In the diagram according to FIG. 10 the graph 33 represents the opening force and graph 34 represents movement of valve element 27. When the valve element is in open position one of passages 17 is connected to passage 23 for supply of water or cleaning liquid, respectively, to the nozzle. According to the invention the valve is constructed so that the armature and the plunger with the rubber ele¬ ment can move in relation to each other and the armature and the plunger with the rubber element are allowed to move a distance at opening of the valve, which is longer than that required in order to obtain the necessary maxi¬ mum flow. In this manner the operating cycle of the valve can be divided into three steps. At opening: the accele¬ ration step (FIG. 7) during which the armature moves without the plunger with the rubber element is moving, the opening step (FIG. 8) during which the flow is changed, and the over-movement step (FIG. 9) which guarantees that bounces when the armature is arrested do not result in a change of the flow. At closing the opera¬ tion is similar with the exception that in the accelera- tion step (FIG. 9) the armature as well as the plunger with the rubber element move without the flow being decreased, and in the over-movement step (FIG. 7) the armature only, not the plunger with the rubber element, are moving. It should be noted that the rubber element need not take up inertia forces from the armature, which provides a longer life of the rubber element. By the function of the valve as described large liquid droplets are avoided at opening and closing. In order to eliminate such disturbances of the spray pattern and damages on the conduit system which may be caused by pressure drops and pressure peaks, an attenuator is provided (FIG. 4) . Recess 24 is connected by a passage 35 with a space inside an elastic dome 36. This dome can expand and retract, respectively, at pressure peaks and pressure drops, respectively. The attenuator can also have two spaces separated by an elas¬ tic membrane; one space is connected to passage 35 and the other space contains a compressible element e.g. a gas or a spring. The attenuator can be connected at any place in the conduit system but preferably should be con¬ nected as close as possible to the valve. In order to obtain a sufficiently low time separa¬ tion during the opening periods of the valves a large amount of hardware (counters etc.) is required. In order to reduce the need of hardware and thus to reduce the cost for the system a method is provided according to the invention, wherein the physical time resolution is increased. By the introduction of a calculation function storing a residual value between two opening operations following one upon the other the mean value of a number of such opening operations can be set with a better reso¬ lution than that of the individual opening operation. Assuming for example that the opening period 17.33 is required for a certain amount of moisture and that the valve can be controlled in steps of integers of the opening period only. Then, the opening sequence 17, 17, 18 is introduced said sequence having a mean value of 17.33. When the wetting cylinder in the printing machine is moistened by spraying it is of greatest importance that the valve controlling the moistening liquid can be opened and closed at high speed and at high timely accuracy. In order to obtain rapid opening of a solenoid valve it is required that there is obtained a rapid increase of the current through the coil of the valve (the force which accelerates the armature is a function of the current) . In order to obtain the same delay and increase of flow when a valve is being opened it is required that current and voltage are controlled according to a predetermined pattern. In the first step of the opening cycle the change of the current is depen¬ dent substantially on the voltage applied, and it is therefore important that this voltage is constant. When the current, graph 37 in FIG.10, has increased to a pre¬ determined value (typically to two or three times the nominal current) which provides the desired accelerating forces, the optimal step is to adjust the current to the predetermined level. In this manner resistance changes in the coil and conductors will not effect the force, graph 33 in FIG. 10, which actuates the armature. Thus, a more controlled opening process will be obtained, graph 34 in FIG. 10, for the valve. When the predetermined current has been reached and then is being maintained for a pre¬ determined time the armature has accelerated to a speed according to graph 38 in FIG.10 and moves towards the arrested position. At this time the air gap (in this case filled with liquid) in the magnetic circuit has also decreased so that the current required to obtain the necessary forces in order to maintain the armature in the inner position thereof, graph 39 in FIG. 10, has decreased considerably. In order not to pass the maximum power loss of the coil, that can be allowed, it is now suitable to reduce the current below the nominal value during the remaining time during which the valve has to be open. When the valve shall be closed the current through the coil must be reduced to zero as rapidly as possible in order that the magnetic force on the armature shall cease and the armature shall return to the rest position thereof by the spring/springs as rapidly as pos¬ sible. A rapid reduction of the current is obtained by reversing the voltage over the coil. In the embodiment described the passages 22 and 23 are connected via the two valves to one and the same nozzle 21 but it is also possible to let one valve control the flow through a nozzle for water and to let the other valve control a nozzle for cleaning liquid; each valve thus is associated with a separate nozzle.

Claims: CLAIMS 1. Device for wetting and/or cleaning cylinders in an offset rotary printing machine having a plate cylinder, an offset cylinder and an impression cylinder, comprising wetting cylinders running against the plate cylinder, a ramp having several nozzles and mounted in parallel to the rotational axes of the cylinders for spraying liquid towards one of the wetting cylinders in atomized condition, and a valve for each nozzle having at least one separate solenoid controlled inlet (16, 17, 24) for liquid and one outlet (22, 23) to the associated nozzle, c h a r a c t e r i z e d in that the solenoid controlled inlet (16, 17, 24) comprises a valve 10 having a solenoid armature (25) which is spring biased towards the closed position of the valve, a valve element displaceably mounted in the solenoid valve said valve element being spring biased towards the closed position of the valve and engaging in this position a valve seat (23, 24), and abutments (30, 31) for limiting the displacement of the valve element in relation to the solenoid armature under influence of the spring bias of the valve element, said abutments being kept separated against the spring bias of the valve element when the valve element engages the valve seat. 2. Device according to claim 1, c h a r a c t e r i z e d in that an expandable and con- tractable attenuating member (34) for equalizing pressure peaks and pressure drops at the operation of the valve communicates with the inlet (16, 17, 24) . 3. Device according to claim 1, c h a r a c t e r i z e d in that the valve (10) is straddling a supply conduit (12) for liquid with the inlet (16, 17, 24) in register with an aperture (14) in the conduit and is clamped against the conduit, a sealing ring (15) surrounding the aperture being provided there¬ between. 4. Method of controlling the device according to claim 1, c h a r a c t e r i z e d in that the solenoid current at opening of the valve after an initial increase at constant voltage over the solenoid is adjusted to a predetermined value which is maintained for a predetermined period, and that the current then is decreased below the predetermined value to be maintained at a lower value while the valve is in open position. 5. Method according to claim 4, c h a r a c t e r i z e d in that the voltage over the solenoid is reversed for rapid decrease of the current to zero at closing of the valve.

Inventors: LARSSON MAGNUS, LARSSON, MAGNUS
Applicants: GRAFISKT TEAM AB, LARSSON MAGNUS, LARSSON, MAGNUS
Application year: 1994
Main classification: B41F7
IPC codes: B41F35